Systems and methods for magnetic resonance imaging

ABSTRACT

The present disclosure provides a system for MRI. The system may obtain a plurality of echo signals relating to a subject that are excited by an MRI pulse sequence applied to the subject. The system may perform a quantitative measurement on the subject based on the plurality of echo signals. The MRI pulse sequence may include a CEST module configured to selectively excite exchangeable protons or exchangeable molecules in the subject, an RF excitation pulse applied after the CEST module configured to excite a plurality of gradient echoes, and one or more refocusing pulses applied after the RF excitation pulse. Each of the refocusing pulses may be configured to excite one or more spin echoes. The one or more spin echoes excited by at least one of the one or more refocusing pulses may include a symmetric spin echo and one or more asymmetric spin echoes.

TECHNICAL FIELD

The present disclosure generally relates to magnetic resonance imaging(MRI), and more particularly, relates to systems and methods forperforming a quantitative measurement on a subject using MRI techniques.

BACKGROUND

MRI systems are widely used in disease diagnosis and/or treatment.Recently, chemical exchange saturation transfer (CEST) MRI has beendeveloped and used to perform various measurements (e.g., a longitudinalrelaxation time (T1) measurement, a transverse relaxation time (T2)measurement, an amide proton transfer (APT) measurement, a myelin waterfraction (MWF) measurement, a pH measurement) on a subject to provide abasis for the disease diagnosis and/or treatment.

SUMMARY

According to one aspect of the present disclosure, a system for MRI isprovided. The system may include at least one storage device including aset of instructions, and at least one processor configured tocommunicate with the at least one storage device. When executing theinstructions, the at least one processor may be configured to direct thesystem to perform the following operations. The at least one processormay be configured to direct the system to obtain a plurality of echosignals relating to a subject. The plurality of echo signals may beexcited by an MRI pulse sequence applied to the subject. The at leastone processor may be configured to direct the system to perform aquantitative measurement on the subject based on the plurality of echosignals. The MRI pulse sequence may include a CEST module, a radiofrequency (RF) excitation pulse applied after the CEST module, and oneor more refocusing pulses applied after the RF excitation pulse. TheCEST module may be configured to selectively excite exchangeable protonsor exchangeable molecules in the subject. The RF excitation pulse may beconfigured to configured to excite a plurality of gradient echoes. Eachof the one or more refocusing pulses may be configured to excite one ormore spin echoes. The one or more spin echoes excited by at least one ofthe one or more refocusing pulses may include a symmetric spin echo andone or more asymmetric spin echoes.

In some embodiments, the one or more refocusing pulses may include atleast three refocusing pulses.

In some embodiments, the one or more spin echoes excited by each of theat least three refocusing pulses comprise a symmetric spin echo. Theperforming a quantitative measurement on the subject based on theplurality of echo signals may include determining an MWF relating to thesubject based on the symmetric spin echoes excited by the at least threerefocusing pulses.

In some embodiments, the performing a quantitative measurement on thesubject based on the plurality of echo signals may include determiningan R2 value and an R2* value relating to the subject based on theplurality of gradient echoes, the one or more asymmetric spin echoesexcited by the at least one refocusing pulse, and the symmetric spinecho excited by the at least one refocusing pulse.

In some embodiments, the at least one processor may be configured todirect the system to determine an R2′ value relating to the subjectbased on the R2 value and the R2* value.

In some embodiments, the at least one processor may be configured todirect the system to determine a coefficient that measures a differencebetween a slice profile corresponding to the RF excitation pulse and aslice profile corresponding to the at least one refocusing pulse basedon the plurality of gradient echoes, the one or more asymmetric spinechoes excited by the at least one refocusing pulse, and the symmetricspin echo excited by the at least one refocusing pulse.

In some embodiments, the plurality of gradient echoes may beR2*-weighted gradient echoes. The one or more asymmetric spin echoesexcited by the at least one refocusing pulse may be R2- and R2*-weightedspin echoes. The symmetric spin-echo excited by the at least onerefocusing pulse may be an R2-weighted spin echo.

In some embodiments, the MRI pulse sequence may further include a fatsuppression module applied between the CEST module and the RF excitationpulse configured to suppress signals from an adipose tissue of thesubject.

In some embodiments, the MRI pulse sequence may be applied to thesubject for multiple times with different saturation frequencies of theCEST module in a plurality of acquisitions. The plurality of echosignals may include a plurality of sets of echo signals each of whichare acquired in one of the plurality of acquisitions. The performing aquantitative measurement on the subject based on the plurality of echosignals may include performing a magnetization transfer asymmetry(MTRasym) analysis on the plurality of sets of echo signals.

In some embodiments, the performing a quantitative measurement on thesubject based on the plurality of echo signals further may includedetermining a pH value relating to the subject based on a result of theMTRasym analysis.

In some embodiments, the at least one processor may be furtherconfigured to direct the system to generate a B0 map based on theplurality of echo signals.

According to another aspect of the present disclosure, a method for MRIis provided. The method may include obtaining a plurality of echosignals relating to a subject and performing a quantitative measurementon the subject based on the plurality of echo signals. The plurality ofecho signals may be excited by an MRI pulse sequence applied to thesubject. The method may also include performing a quantitativemeasurement on the subject based on the plurality of echo signals. TheMRI pulse sequence may include a CEST module, an RF excitation pulseapplied after the CEST module, and one or more refocusing pulses appliedafter the RF excitation pulse. The CEST module may be configured toselectively excite exchangeable protons or exchangeable molecules in thesubject. The RF excitation pulse may be configured to configured toexcite a plurality of gradient echoes. Each of the one or morerefocusing pulses may be configured to excite one or more spin echoes.The one or more spin echoes excited by at least one of the one or morerefocusing pulses may include a symmetric spin echo and one or moreasymmetric spin echoes.

According to another aspect of the present disclosure, a non-transitoryreadable medium including at least one set of instructions for MRI isprovided. When accessed by at least one processor, the at least one setof instructions may direct the at least one processor to perform amethod. The method may include obtaining a plurality of echo signalsrelating to a subject and performing a quantitative measurement on thesubject based on the plurality of echo signals. The plurality of echosignals may be excited by an MRI pulse sequence applied to the subject.The method may also include performing a quantitative measurement on thesubject based on the plurality of echo signals. The MRI pulse sequencemay include a CEST module, an RF excitation pulse applied after the CESTmodule, and one or more refocusing pulses applied after the RFexcitation pulse. The CEST module may be configured to selectivelyexcite exchangeable protons or exchangeable molecules in the subject.The RF excitation pulse may be configured to configured to excite aplurality of gradient echoes. Each of the one or more refocusing pulsesmay be configured to excite one or more spin echoes. The one or morespin echoes excited by at least one of the one or more refocusing pulsesmay include a symmetric spin echo and one or more asymmetric spinechoes.

Additional features will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the artupon examination of the following and the accompanying drawings or maybe learned by production or operation of the examples. The features ofthe present disclosure may be realized and attained by practice or useof various aspects of the methodologies, instrumentalities, andcombinations set forth in the detailed examples discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is further described in terms of exemplaryembodiments. These exemplary embodiments are described in detail withreference to the drawings. The drawings are not to scale. Theseembodiments are non-limiting exemplary embodiments, in which likereference numerals represent similar structures throughout the severalviews of the drawings, and wherein:

FIG. 1 is a schematic diagram illustrating an exemplary MRI systemaccording to some embodiments of the present disclosure;

FIG. 2 is a schematic diagram illustrating an exemplary MR scanneraccording to some embodiments of the present disclosure;

FIG. 3 is a schematic diagram illustrating exemplary hardware and/orsoftware components of a computing device according to some embodimentsof the present disclosure;

FIG. 4 is a schematic diagram illustrating exemplary hardware and/orsoftware components of a mobile device according to some embodiments ofthe present disclosure;

FIG. 5 is a block diagram illustrating an exemplary processing deviceaccording to some embodiments of the present disclosure;

FIG. 6 is a flowchart illustrating an exemplary process for performing aquantitative measurement on a subject according to some embodiments ofthe present disclosure;

FIG. 7 is a schematic diagram illustrating an exemplary pulse sequenceaccording to some embodiments of the present disclosure;

FIG. 8 is a schematic diagram illustrating an exemplary CEST moduleaccording to some embodiments of the present disclosure;

FIG. 9 is a schematic diagram illustrating an exemplary GRASE pulsesequence according to some embodiments of the present disclosure;

FIG. 10 is a schematic diagram illustrating an exemplary GRASE pulsesequence according to some embodiments of the present disclosure;

FIG. 11 is a schematic diagram illustrating an exemplary GRASE pulsesequence according to some embodiments of the present disclosure;

FIG. 12 is a schematic diagram illustrating an exemplary GRASE pulsesequence according to some embodiments of the present disclosure; and

FIGS. 13A-13C are schematic diagrams illustrating exemplary z-spectra ofa cancer tissue and a healthy tissue of the subject according to someembodiments of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant disclosure. However, it should be apparent to those skilledin the art that the present disclosure may be practiced without suchdetails. In other instances, well-known methods, procedures, systems,components, and/or circuitry have been described at a relativelyhigh-level, without detail, in order to avoid unnecessarily obscuringaspects of the present disclosure. Various modifications to thedisclosed embodiments will be readily apparent to those skilled in theart, and the general principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present disclosure. Thus, the present disclosure is not limitedto the embodiments shown, but to be accorded the widest scope consistentwith the claims.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise,”“comprises,” and/or “comprising,” “include,” “includes,” and/or“including,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

It will be understood that the term “system,” “engine,” “unit,”“module,” and/or “block” used herein are one method to distinguishdifferent components, elements, parts, sections or assembly of differentlevels in ascending order. However, the terms may be displaced byanother expression if they achieve the same purpose.

Generally, the word “module,” “unit,” or “block,” as used herein, refersto logic embodied in hardware or firmware, or to a collection ofsoftware instructions. A module, a unit, or a block described herein maybe implemented as software and/or hardware and may be stored in any typeof non-transitory computer-readable medium or another storage device. Insome embodiments, a software module/unit/block may be compiled andlinked into an executable program. It will be appreciated that softwaremodules can be callable from other modules/units/blocks or fromthemselves, and/or may be invoked in response to detected events orinterrupts. Software modules/units/blocks configured for execution oncomputing devices (e.g., processor 310 as illustrated in FIG. 3) may beprovided on a computer-readable medium, such as a compact disc, adigital video disc, a flash drive, a magnetic disc, or any othertangible medium, or as a digital download (and can be originally storedin a compressed or installable format that needs installation,decompression, or decryption prior to execution). Such software code maybe stored, partially or fully, on a storage device of the executingcomputing device, for execution by the computing device. Softwareinstructions may be embedded in firmware, such as an EPROM. It will befurther appreciated that hardware modules/units/blocks may be includedin connected logic components, such as gates and flip-flops, and/or canbe included of programmable units, such as programmable gate arrays orprocessors. The modules/units/blocks or computing device functionalitydescribed herein may be implemented as software modules/units/blocks,but may be represented in hardware or firmware. In general, themodules/units/blocks described herein refer to logicalmodules/units/blocks that may be combined with othermodules/units/blocks or divided into sub-modules/sub-units/sub-blocksdespite their physical organization or storage. The description may beapplicable to a system, an engine, or a portion thereof.

It will be understood that when a unit, engine, module or block isreferred to as being “on,” “connected to,” or “coupled to,” anotherunit, engine, module, or block, it may be directly on, connected orcoupled to, or communicate with the other unit, engine, module, orblock, or an intervening unit, engine, module, or block may be present,unless the context clearly indicates otherwise. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items. The term “image” in the present disclosure isused to collectively refer to image data (e.g., scan data, projectiondata) and/or images of various forms, including a two-dimensional (2D)image, a three-dimensional (3D) image, a four-dimensional (4D), etc. Theterm “pixel” and “voxel” in the present disclosure are usedinterchangeably to refer to an element of an image.

It will be understood that, although the terms “first,” “second,”“third,” etc., may be used herein to describe various elements, theseelements should not be limited by these terms. These terms are only usedto distinguish one element from another. For example, a first elementcould be termed a second element, and, similarly, a second element couldbe termed a first element, without departing from the scope of exampleembodiments of the present invention.

These and other features, and characteristics of the present disclosure,as well as the methods of operation and functions of the relatedelements of structure and the combination of parts and economies ofmanufacture, may become more apparent upon consideration of thefollowing description with reference to the accompanying drawings, allof which form a part of this disclosure. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended to limit thescope of the present disclosure. It is understood that the drawings arenot to scale.

Provided herein are systems and methods for non-invasive biomedicalimaging, such as for disease diagnostic or research purposes. While thesystems and methods disclosed in the present disclosure are describedprimarily regarding an MRI system. It should be understood that this isonly for illustration purposes. The systems and methods of the presentdisclosure may be applied to any other kind of imaging system. In someembodiments, the imaging system may include a single modality imagingsystem and/or a multi-modality imaging system. The single modalityimaging system may include, for example, the MRI system. Themulti-modality imaging system may include, for example, an X-rayimaging-magnetic resonance imaging (X-ray-MRI) system, a single photonemission computed tomography-magnetic resonance imaging (SPECT-MRI)system, a digital subtraction angiography-magnetic resonance imaging(DSA-MRI) system, a computed tomography-magnetic resonance imaging(MRI-CT) system, a positron emission tomography-magnetic resonanceimaging (PET-MRI) system, etc.

CEST imaging is a magnetic resonance imaging contrast approach in whichexogenous or endogenous compounds including either exchangeable protonsor exchangeable molecules are selectively saturated and after thetransfer of this saturation, detected indirectly through a water signalwith enhanced sensitivity. CEST imaging has been used to produce variousquantitative results of a subject (e.g., a patient or a portionthereof). For example, CEST imaging may be used to acquire an APTweighted image, a pH weighted image, a signal decay rate (R2) map, atransverse relaxation rate (R2*) map, a reversible transverse relaxationrate (R2′) map, an MWF map, a B0 map, or the like, or any combinationthereof, of the subject. Conventionally, information relating todifferent quantitative parameters may need to be acquired separately byapplying different CEST pulse sequences to the subject. For example,information relating to APT and pH of the subject may be obtained byapplying a first MRI pulse to the subject, and information relating toR2, R2*, R2′, and MWF may be obtained by applying a second MRI pulsedifferent from the first MRI pulse to the subject. This may waste timeand human sources, and reduce the efficiency of CEST imaging and diseasediagnosis performed based thereon.

In order to improve the efficiency of CEST imaging, an aspect of thepresent disclosure provides systems and methods that can be used tosimultaneously acquire information relating to a plurality ofquantitative parameters of the subject. The systems may obtain aplurality of echo signals relating to a subject. The plurality of echosignals may be excited by an MRI pulse sequence applied to the subject.The MRI system may perform a quantitative measurement on the subjectbased on the plurality of echo signals. For example, the quantitativemeasurement may involve determining a plurality of quantitativeparameters, such as an R2 value, an R2* value, an R2′ value, an MWF, anAPT, pH, a B0 value, or the like, or any combination thereof, relatingto the subject.

The MRI pulse sequence may have a specially designed configuration thatenables simultaneous acquisition of the plurality of quantitativeparameters. Merely by way of example, the MRI pulse sequence may includea CEST module configured to selectively excite exchangeable protons orexchangeable molecules in the subject. The MRI pulse sequence may alsoinclude an RF excitation pulse applied after the CEST module configuredto excite a plurality of gradient echoes. The MRI pulse sequence mayfurther include one or more refocusing pulses applied after the RFexcitation pulse. Each of the one or more refocusing pulses may beconfigured to excite one or more spin echoes. The one or more spinechoes excited by at least one of the one or more refocusing pulses mayinclude a symmetric spin echo and one or more asymmetric spin echoes. Byapplying the MRI pulse sequence with the specially designedconfiguration, more information relating to the subject may be acquiredthrough one acquisition to achieve the quantitative measurement relatingto multiple quantitative parameters. In this way, the systems disclosedin the present disclosure may improve the accuracy of the diseasediagnosis (e.g., by providing multidimensional information) and theefficiency of MRI (e.g., by reducing a scanning time of the subject).

FIG. 1 is a schematic diagram illustrating an exemplary MRI system 100according to some embodiments of the present disclosure. As shown inFIG. 1, the MRI system 100 may include an MR scanner 110 (or referred toas an MRI scanner), a processing device 120, a storage device 130, oneor more terminals 140, and a network 150. In some embodiments, the MRscanner 110, the processing device 120, the storage device 130, and/orthe terminal(s) 140 may be connected to and/or communicate with eachother via a wireless connection, a wired connection, or a combinationthereof. The connections between the components in the MRI system 100may be variable. For example, the MR scanner 110 may be connected to theprocessing device 120 through the network 150. As another example, theMR scanner 110 may be connected to the processing device 120 directly.

The MR scanner 110 may be configured to scan a subject (or a part of thesubject) to acquire image data, such as echo signals (or MR signals)associated with the subject. For example, the MR scanner 110 may detecta plurality of echo signals by applying an MR pulse sequence on thesubject. In some embodiments, the MR scanner 110 may include, forexample, a main magnet, a gradient coil (or also referred to a spatialencoding coil), a radio frequency (RF) coil, etc., as described inconnection with FIG. 2. In some embodiments, the MR scanner 110 may be apermanent magnet MR scanner, a superconducting electromagnet MR scanner,or a resistive electromagnet MR scanner, etc., according to types of themain magnet. In some embodiments, the MR scanner 110 may be a high-fieldMR scanner, a mid-field MR scanner, and a low-field MR scanner, etc.,according to the intensity of the magnetic field.

The subject scanned by the MR scanner 110 may be biological ornon-biological. For example, the subject may include a patient, aman-made object, etc. As another example, the subject may include aspecific portion, organ, tissue, and/or a physical point of the patient.Merely by way of example, the subject may include head, brain, neck,body, shoulder, arm, thorax, cardiac, stomach, blood vessel, softtissue, knee, feet, or the like, or a combination thereof.

For illustration purposes, a coordinate system 160 including an X-axis,a Y-axis, and a Z-axis is provided in FIG. 1. The X-axis and the Z-axisshown in FIG. 1 may be horizontal, and the Y-axis may be vertical. Asillustrated, the positive X direction along the X-axis may be from theright side to the left side of the MR scanner 110 seen from thedirection facing the front of the MR scanner 110; the positive Ydirection along the Y-axis shown in FIG. 1 may be from the lower part tothe upper part of the MR scanner 110; the positive Z direction along theZ-axis shown in FIG. 1 may refer to a direction in which the subject ismoved out of the scanning channel (or referred to as the bore) of the MRscanner 110. More description of the MR scanner 110 may be foundelsewhere in the present disclosure. See, e.g., FIG. 2 and thedescription thereof.

The processing device 120 may process data and/or information obtainedfrom the MR scanner 110, the storage device 130, and/or the terminal(s)140. For example, the MR scanner 110 may excite a plurality of echosignals relating to the subject by scanning the subject. The processingdevice 120 may obtain the echo signals from the MR scanner 110, andperform a quantitative measurement on the subject based on the echosignals. In some embodiments, the processing device 120 may be a singleserver or a server group. The server group may be centralized ordistributed. In some embodiments, the processing device 120 may be localor remote. For example, the processing device 120 may access informationand/or data from the MR scanner 110, the storage device 130, and/or theterminal(s) 140 via the network 150. As another example, the processingdevice 120 may be directly connected to the MR scanner 110, theterminal(s) 140, and/or the storage device 130 to access informationand/or data. In some embodiments, the processing device 120 may beimplemented on a cloud platform. For example, the cloud platform mayinclude a private cloud, a public cloud, a hybrid cloud, a communitycloud, a distributed cloud, an inter-cloud, a multi-cloud, or the like,or a combination thereof. In some embodiments, the processing device 120may be implemented by a computing device 300 having one or morecomponents as described in connection with FIG. 3.

The storage device 130 may store data, instructions, and/or any otherinformation. In some embodiments, the storage device 130 may store dataobtained from the MR scanner 110, the processing device 120, and/or theterminal(s) 140. In some embodiments, the storage device 130 may storedata and/or instructions that the processing device 120 may execute oruse to perform exemplary methods described in the present disclosure. Insome embodiments, the storage device 130 may include a mass storagedevice, a removable storage device, a volatile read-and-write memory, aread-only memory (ROM), or the like, or a combination thereof. Exemplarymass storage devices may include a magnetic disk, an optical disk, asolid-state drive, etc. Exemplary removable storage devices may includea flash drive, a floppy disk, an optical disk, a memory card, a zipdisk, a magnetic tape, etc. Exemplary volatile read-and-write memory mayinclude a random access memory (RAM). Exemplary RAM may include adynamic RAM (DRAM), a double date rate synchronous dynamic RAM (DDRSDRAM), a static RAM (SRAM), a thyristor RAM (T-RAM), a zero-capacitorRAM (Z-RAM), etc. Exemplary ROM may include a mask ROM (MROM), aprogrammable ROM (PROM), an erasable programmable ROM (EPROM), anelectrically erasable programmable ROM (EEPROM), a compact disk ROM(CD-ROM), a digital versatile disk ROM, etc. In some embodiments, thestorage device 130 may be implemented on a cloud platform as describedelsewhere in the disclosure.

In some embodiments, the storage device 130 may be connected to thenetwork 150 to communicate with one or more other components in the MRIsystem 100 (e.g., the MR scanner 110, the processing device 120, and/orthe terminal(s) 140). One or more components of the MRI system 100 mayaccess the data or instructions stored in the storage device 130 via thenetwork 150. In some embodiments, the storage device 130 may be part ofthe processing device 120 or the terminal(s) 140.

The terminal(s) 140 may be configured to enable a user interactionbetween a user and the MRI system 100. For example, the terminal(s) 140may receive an instruction to cause the MR scanner 110 to scan thesubject from the user. As another example, the terminal(s) 140 mayreceive a processing result (e.g., an R2 map and an R2* map relating tothe subject) from the processing device 120 and display the processingresult to the user. In some embodiments, the terminal(s) 140 may beconnected to and/or communicate with the MR scanner 110, the processingdevice 120, and/or the storage device 130. In some embodiments, theterminal(s) 140 may include a mobile device 140-1, a tablet computer140-2, a laptop computer 140-3, or the like, or a combination thereof.For example, the mobile device 140-1 may include a mobile phone, apersonal digital assistant (PDA), a gaming device, a navigation device,a point of sale (POS) device, a laptop, a tablet computer, a desktop, orthe like, or a combination thereof. In some embodiments, the terminal(s)140 may include an input device, an output device, etc. The input devicemay include alphanumeric and other keys that may be input via akeyboard, a touch screen (for example, with haptics or tactilefeedback), a speech input, an eye tracking input, a brain monitoringsystem, or any other comparable input mechanism. The input informationreceived through the input device may be transmitted to the processingdevice 120 via, for example, a bus, for further processing. Other typesof the input device may include a cursor control device, such as amouse, a trackball, or cursor direction keys, etc. The output device mayinclude a display, a speaker, a printer, or the like, or a combinationthereof. In some embodiments, the terminal(s) 140 may be part of theprocessing device 120 or the MR scanner 110.

The network 150 may include any suitable network that can facilitate theexchange of information and/or data for the MRI system 100. In someembodiments, one or more components of the MRI system 100 (e.g., the MRscanner 110, the processing device 120, the storage device 130, theterminal(s) 140, etc.) may communicate information and/or data with oneor more other components of the MRI system 100 via the network 150. Forexample, the processing device 120 may obtain image data (e.g., an echosignal) from the MR scanner 110 via the network 150. As another example,the processing device 120 may obtain user instructions from theterminal(s) 140 via the network 150. The network 150 may include apublic network (e.g., the Internet), a private network (e.g., a localarea network (LAN), a wide area network (WAN)), etc.), a wired network(e.g., an Ethernet network), a wireless network (e.g., an 802.11network, a Wi-Fi network, etc.), a cellular network (e.g., a Long TermEvolution (LTE) network), a frame relay network, a virtual privatenetwork (“VPN”), a satellite network, a telephone network, routers,hubs, switches, server computers, or the like, or a combination thereof.For example, the network 150 may include a cable network, a wirelinenetwork, a fiber-optic network, a telecommunications network, anintranet, a wireless local area network (WLAN), a metropolitan areanetwork (MAN), a public telephone switched network (PSTN), a Bluetooth™network, a ZigBee™ network, a near field communication (NFC) network, orthe like, or a combination thereof. In some embodiments, the network 150may include one or more network access points. For example, the network150 may include wired and/or wireless network access points such as basestations and/or internet exchange points through which one or morecomponents of the MRI system 100 may be connected to the network 150 toexchange data and/or information.

This description is intended to be illustrative, and not to limit thescope of the present disclosure. Many alternatives, modifications, andvariations will be apparent to those skilled in the art. The features,structures, methods, and characteristics of the exemplary embodimentsdescribed herein may be combined in various ways to obtain additionaland/or alternative exemplary embodiments. In some embodiments, the MRIsystem 100 may include one or more additional components and/or one ormore components described above may be omitted. Additionally oralternatively, two or more components of the MRI system 100 may beintegrated into a single component. For example, the processing device120 may be integrated into the MR scanner 110. As another example, acomponent of the MRI system 100 may be replaced by another componentthat can implement the functions of the component. In some embodiments,the storage device 130 may be a data storage including cloud computingplatforms, such as a public cloud, a private cloud, a community andhybrid cloud, etc. However, those variations and modifications do notdepart the scope of the present disclosure.

FIG. 2 is a schematic diagram illustrating an exemplary MR scanner 110according to some embodiments of the present disclosure. One or morecomponents of the MR scanner 110 are illustrated in FIG. 2. Asillustrated, main magnet 201 may generate a first magnetic field (orreferred to as a main magnetic field) that may be applied to a subjectexposed inside the field. The main magnet 201 may include a resistivemagnet or a superconductive magnet that both need a power supply (notshown) for operation. Alternatively, the main magnet 201 may include apermanent magnet. The main magnet 201 may include a bore that thesubject is placed within. The main magnet 201 may also control thehomogeneity of the generated main magnetic field. Some shim coils may bein the main magnet 201. The shim coils placed in the gap of the mainmagnet 201 may compensate for the inhomogeneity of the magnetic field ofthe main magnet 201. The shim coils may be energized by a shim powersupply.

Gradient coils 202 may be located inside the main magnet 201. Thegradient coils 202 may generate a second magnetic field (or referred toas a gradient field, including gradient fields Gx, Gy, and Gz). Thesecond magnetic field may be superimposed on the main field generated bythe main magnet 201 and distort the main field so that the magneticorientations of the protons of a subject may vary as a function of theirpositions inside the gradient field, thereby encoding spatialinformation into echo signals generated by the region of the subjectbeing imaged. The gradient coils 202 may include X coils (e.g.,configured to generate the gradient field Gx corresponding to the Xdirection), Y coils (e.g., configured to generate the gradient field Gycorresponding to the Y direction), and/or Z coils (e.g., configured togenerate the gradient field Gz corresponding to the Z direction) (notshown in FIG. 2). In some embodiments, the Z coils may be designed basedon circular (Maxwell) coils, while the X coils and the Y coils may bedesigned on the basis of the saddle (Golay) coil configuration. Thethree sets of coils may generate three different magnetic fields thatare used for position encoding. The gradient coils 202 may allow spatialencoding of echo signals for image construction. The gradient coils 202may be connected with one or more of an X gradient amplifier 204, a Ygradient amplifier 205, or a Z gradient amplifier 206. One or more ofthe three amplifiers may be connected to a waveform generator 216. Thewaveform generator 216 may generate gradient waveforms that are appliedto the X gradient amplifier 204, the Y gradient amplifier 205, and/orthe Z gradient amplifier 206. An amplifier may amplify a waveform. Anamplified waveform may be applied to one of the coils in the gradientcoils 202 to generate a magnetic field in the X-axis, the Y-axis, or theZ-axis, respectively. The gradient coils 202 may be designed for eithera close-bore MR scanner or an open-bore MR scanner. In some instances,all three sets of coils of the gradient coils 202 may be energized andthree gradient fields may be generated thereby. In some embodiments ofthe present disclosure, the X coils and Y coils may be energized togenerate the gradient fields in the X direction and the Y direction. Asused herein, the X-axis, the Y-axis, the Z-axis, the X direction, the Ydirection, and the Z direction in the description of FIG. 2 are the sameas or similar to those described in FIG. 1.

In some embodiments, radio frequency (RF) coils 203 may be locatedinside the main magnet 201 and serve as transmitters, receivers, orboth. The RF coils 203 may be in connection with RF electronics 209 thatmay be configured or used as one or more integrated circuits (ICs)functioning as a waveform transmitter and/ora waveform receiver. The RFelectronics 209 may be connected to a radiofrequency power amplifier(RFPA) 207 and an analog-to-digital converter (ADC) 208.

When used as transmitters, the RF coils 203 may generate RF signals thatprovide a third magnetic field that is utilized to generate echo signalsrelated to the region of the subject being imaged. The third magneticfield may be perpendicular to the main magnetic field. The waveformgenerator 216 may generate an RF excitation pulse. The RF excitationpulse may be amplified by the RFPA 207, processed by the RF electronics209, and applied to the RF coils 203 to generate the RF signals inresponse to a powerful current generated by the RF electronics 209 basedon the amplified RF excitation pulse.

When used as receivers, the RF coils may be responsible for detectingecho signals. After excitation, the echo signals generated by thesubject may be sensed by the RF coils 203. The receive amplifier thenmay receive the sensed echo signals from the RF coils 203, amplify thesensed echo signals, and provide the amplified echo signals to the ADC208. The ADC 208 may transform the echo signals from analog signals todigital signals. The digital echo signals then may be sent to theprocessing device 120 for sampling.

In some embodiments, the gradient coils 202 and the RF coils 203 may becircumferentially positioned with respect to the subject. It isunderstood by those skilled in the art that the main magnet 201, thegradient coils 202, and the RF coils 203 may be situated in a variety ofconfigurations around the subject.

In some embodiments, the RFPA 207 may amplify an RF excitation pulse(e.g., the power of the RF excitation pulse, the voltage of the RFexcitation pulse) such that an amplified RF excitation pulse isgenerated to drive the RF coils 203. The RFPA 207 may include atransistor-based RFPA, a vacuum tube-based RFPA, or the like, or anycombination thereof. The transistor-based RFPA may include one or moretransistors. The vacuum tube-based RFPA may include a triode, a tetrode,a klystron, or the like, or any combination thereof. In someembodiments, the RFPA 207 may include a linear RFPA, or a nonlinearRFPA. In some embodiments, the RFPA 207 may include one or more RFPAs.

In some embodiments, the MR scanner 110 may further include a subjectpositioning system (not shown). The subject positioning system mayinclude a subject cradle and a transport device. The subject may beplaced on the subject cradle and be positioned by the transport devicewithin the bore of the main magnet 201.

MRI systems (e.g., the MRI system 100 disclosed in the presentdisclosure) may be commonly used to obtain an interior image from apatient for a particular region of interest (ROI) that can be used forthe purposes of, e.g., diagnosis, treatment, or the like, or acombination thereof. MRI systems include a main magnet (e.g., the mainmagnet 201) assembly for providing a strong uniform main magnetic fieldto align the individual magnetic moments of the H atoms within thepatient's body. During this process, the H atoms oscillate around theirmagnetic poles at their characteristic Larmor frequency. If the tissueis subjected to an additional magnetic field, which is tuned to theLarmor frequency, the H atoms absorb additional energy, which rotatesthe net aligned moment of the H atoms. The additional magnetic field maybe provided by an RF excitation signal (e.g., the RF signal generated bythe RF coils 203). When the additional magnetic field is removed, themagnetic moments of the H atoms rotate back into alignment with the mainmagnetic field thereby emitting an echo signal. The echo signal isreceived and processed to form an MR image. T1 relaxation may be theprocess by which the net magnetization grows/returns to its initialmaximum value parallel to the main magnetic field. T1 may be the timeconstant for regrowth of longitudinal magnetization (e.g., along themain magnetic field). T2 relaxation may be the process by which thetransverse components of magnetization decay or dephase. T2 may be thetime constant for decay/dephasing of transverse magnetization.

If the main magnetic field is uniform across the entire body of thepatient, then the RF excitation signal may excite all of the H atoms inthe sample non-selectively. Accordingly, in order to image a particularportion of the patient's body, magnetic field gradients Gx, Gy, and Gz(e.g., generated by the gradient coils 202) in the x, y, and zdirections, having a particular timing, frequency, and phase, may besuperimposed on the uniform magnetic field such that the RF excitationsignal excites the H atoms in a desired slice of the patient's body, andunique phase and frequency information is encoded in the echo signaldepending on the location of the H atoms in the “image slice.”

Typically, portions of the patient's body to be imaged are scanned by asequence of measurement cycles in which the RF excitation signals andthe magnetic field gradients Gx, Gy and Gz vary according to an MRIimaging protocol that is being used. A protocol may be designed for oneor more tissues to be imaged, diseases, and/or clinical scenarios. Aprotocol may include a certain number of pulse sequences oriented indifferent planes and/or with different parameters. The pulse sequencesmay include spin echo sequences, gradient echo sequences, diffusionsequences, inversion recovery sequences, or the like, or any combinationthereof. For instance, the spin echo sequences may include a fast spinecho (FSE) pulse sequence, a turbo spin echo (TSE) pulse sequence, arapid acquisition with relaxation enhancement (RARE) pulse sequence, ahalf-Fourier acquisition single-shot turbo spin-echo (HASTE) pulsesequence, a turbo gradient spin echo (TGSE) pulse sequence, or the like,or any combination thereof. As another example, the gradient echosequences may include a balanced steady-state free precession (bSSFP)pulse sequence, a spoiled gradient echo (GRE) pulse sequence, and anecho planar imaging (EPI) pulse sequence, a steady state free precession(SSFP), or the like, or any combination thereof. The protocol may alsoinclude information regarding image contrast and/or ratio, an ROI, slicethickness, an imaging type (e.g., T1 weighted imaging, T2 weightedimaging, proton density weighted imaging, etc.), T1, T2, an echo type(spin echo, fast spin echo (FSE), fast recovery FSE, single shot FSE,gradient recalled echo, fast imaging with stead-state procession, and soon), a flip angle value, acquisition time (TA), echo time (TE),repetition time (TR), echo train length (ETL), the number of phases, thenumber of excitations (NEX), inversion time, bandwidth (e.g., RFreceiver bandwidth, RF transmitter bandwidth, etc.), or the like, or anycombination thereof. For each MRI scan, the resulting echo signals maybe digitized and processed to reconstruct an image in accordance withthe MRI imaging protocol that is used.

FIG. 3 is a schematic diagram illustrating exemplary hardware and/orsoftware components of a computing device 300 according to someembodiments of the present disclosure. The computing device 300 may beused to implement any component of the MRI system 100 as describedherein. For example, the processing device 120 and/or the terminal 140may be implemented on the computing device 300, respectively, via itshardware, software program, firmware, or a combination thereof. Althoughonly one such computing device is shown, for convenience, the computerfunctions relating to the MRI system 100 as described herein may beimplemented in a distributed fashion on a number of similar platforms,to distribute the processing load.

As illustrated in FIG. 3, the computing device 300 may include aprocessor 310, a storage 320, an input/output (I/O) 330, and acommunication port 340.

The processor 310 may execute computer instructions (e.g., program code)and perform functions of the processing device 120 in accordance withtechniques described herein. The computer instructions may include, forexample, routines, programs, objects, components, data structures,procedures, modules, and functions, which perform particular functionsdescribed herein. For example, the processor 310 may process image dataobtained from the MR scanner 110, the terminal(s) 140, the storagedevice 130, and/or any other component of the MRI system 100. In someembodiments, the processor 310 may include one or more hardwareprocessors, such as a microcontroller, a microprocessor, a reducedinstruction set computer (RISC), an application specific integratedcircuits (ASICs), an application-specific instruction-set processor(ASIP), a central processing unit (CPU), a graphics processing unit(GPU), a physics processing unit (PPU), a microcontroller unit, adigital signal processor (DSP), a field programmable gate array (FPGA),an advanced RISC machine (ARM), a programmable logic device (PLD), anycircuit or processor capable of executing one or more functions, or thelike, or any combinations thereof.

Merely for illustration, only one processor is described in thecomputing device 300. However, it should be noted that the computingdevice 300 in the present disclosure may also include multipleprocessors, thus operations and/or method operations that are performedby one processor as described in the present disclosure may also bejointly or separately performed by the multiple processors. For example,if in the present disclosure the processor of the computing device 300executes both operation A and operation B, it should be understood thatoperation A and operation B may also be performed by two or moredifferent processors jointly or separately in the computing device 300(e.g., a first processor executes operation A and a second processorexecutes operation B, or the first and second processors jointly executeoperations A and B).

The storage 320 may store data/information obtained from the MR scanner110, the terminal(s) 140, the storage device 130, and/or any othercomponent of the MRI system 100. In some embodiments, the storage 320may include a mass storage device, a removable storage device, avolatile read-and-write memory, a read-only memory (ROM), or the like,or any combination thereof. In some embodiments, the storage 320 maystore one or more programs and/or instructions to perform exemplarymethods described in the present disclosure. For example, the storage320 may store a program for the processing device 120 to execute forCEST imaging.

The I/O 330 may input and/or output signals, data, information, etc. Insome embodiments, the I/O 330 may enable a user interaction with theprocessing device 120. In some embodiments, the I/O 330 may include aninput device and an output device. The input device may includealphanumeric and other keys that may be input via a keyboard, a touchscreen (for example, with haptics or tactile feedback), a speech input,an eye tracking input, a brain monitoring system, or any othercomparable input mechanism. The input information received through theinput device may be transmitted to another component (e.g., theprocessing device 120) via, for example, a bus, for further processing.Other types of the input device may include a cursor control device,such as a mouse, a trackball, or cursor direction keys, etc. The outputdevice may include a display (e.g., a liquid crystal display (LCD), alight-emitting diode (LED)-based display, a flat panel display, a curvedscreen, a television device, a cathode ray tube (CRT), a touch screen),a speaker, a printer, or the like, or a combination thereof.

The communication port 340 may be connected to a network (e.g., thenetwork 150) to facilitate data communications. The communication port340 may establish connections between the processing device 120 and theMR scanner 110, the terminal(s) 140, and/or the storage device 130. Theconnection may be a wired connection, a wireless connection, any othercommunication connection that can enable data transmission and/orreception, and/or any combination of these connections. The wiredconnection may include, for example, an electrical cable, an opticalcable, a telephone wire, or the like, or any combination thereof. Thewireless connection may include, for example, a Bluetooth™ link, aWi-Fi™ link, a WiMax™ link, a WLAN link, a ZigBee™ link, a mobilenetwork link (e.g., 3G, 4G, 5G), or the like, or a combination thereof.In some embodiments, the communication port 340 may be and/or include astandardized communication port, such as RS232, RS485, etc. In someembodiments, the communication port 340 may be a specially designedcommunication port. For example, the communication port 340 may bedesigned in accordance with the digital imaging and communications inmedicine (DICOM) protocol.

FIG. 4 is a schematic diagram illustrating exemplary hardware and/orsoftware components of a mobile device 400 according to some embodimentsof the present disclosure. In some embodiments, one or more components(e.g., a terminal 140 and/or the processing device 120) of the MRIsystem 100 may be implemented on the mobile device 400.

As illustrated in FIG. 4, the mobile device 400 may include acommunication platform 410, a display 420, a graphics processing unit(GPU) 430, a central processing unit (CPU) 440, an I/O 450, a memory460, and a storage 490. In some embodiments, any other suitablecomponent, including but not limited to a system bus or a controller(not shown), may also be included in the mobile device 400. In someembodiments, a mobile operating system 470 (e.g., iOS™, Android™,Windows Phone™) and one or more applications 480 may be loaded into thememory 460 from the storage 490 in order to be executed by the CPU 440.The applications 480 may include a browser or any other suitable mobileapps for receiving and rendering information relating to the MRI system100. User interactions with the information stream may be achieved viathe I/O 450 and provided to the processing device 120 and/or othercomponents of the MRI system 100 via the network 150.

To implement various modules, units, and their functionalities describedin the present disclosure, computer hardware platforms may be used asthe hardware platform(s) for one or more of the elements describedherein. A computer with user interface elements may be used to implementa personal computer (PC) or any other type of work station or terminaldevice. A computer may also act as a server if appropriately programmed.

FIG. 5 is a block diagram illustrating an exemplary processing device120 according to some embodiments of the present disclosure. As shown inFIG. 5, the processing device 120 may include an obtaining module 501and a quantitative measurement module 502.

The obtaining module 501 may be configured to obtain a plurality of echosignals relating to a subject (e.g., a patient). The plurality of echosignals may be excited by an MRI pulse sequence applied to the subject.In some embodiments, the MRI pulse sequence may include a CEST module, afat suppression module, an RF excitation pulse, one or more refocusingpulses, or the like, or any combination thereof. The CEST module may beconfigured to selectively excite exchangeable protons or exchangeablemolecules in the subject. The fat suppression module may be appliedbetween the CEST module and the RF excitation pulse and configured tosuppress signals from adipose tissue of the subject. The RF excitationpulse may be applied after the fat suppression module and configured toexcite a plurality of gradient echoes. The refocusing pulses may beapplied after the RF excitation pulse, wherein each refocusing pulse maybe configured to excite one or more spin echoes. More descriptionsregarding the obtaining of the echo signals may be found elsewhere inthe present disclosure. See, e.g., operation 601 and relevantdescriptions thereof.

The quantitative measurement module 502 may be configured to perform aquantitative measurement on the subject based on the plurality of echosignals. The quantitative measurement performed on the subject mayinclude determining a quantitative parameter relating to the subject,generating a quantitative map of the subject, generating a specificimage reflecting a physiological property of the subject, and/or anyother measurement that can evaluate a characteristic of the subject. Insome embodiments, the quantitative measurement module 502 may perform aquantitative measurement relating to a relaxation parameter (e.g., T1,T2, T2*, or T2′), MWF, APT, pH, and B0 of the subject. More descriptionsregarding the quantitative measurement on the subject may be foundelsewhere in the present disclosure. See, e.g., operation 602 andrelevant descriptions thereof.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teachings ofthe present disclosure. However, those variations and modifications donot depart from the scope of the present disclosure. For example, theprocessing device 120 may include one or more additional modules, suchas a storage module (not shown) for storing data. As another example,one or more modules of the processing device 120 described above may beomitted. Additionally or alternatively, two or more modules of theprocessing device 120 may be integrated into a single component. Amodule of the processing device 120 may be divided into two or moreunits.

FIG. 6 is a flowchart illustrating an exemplary process for performing aquantitative measurement on a subject according to some embodiments ofthe present disclosure. In some embodiments, process 600 may be executedby the MRI system 100. For example, the process 600 may be implementedas a set of instructions (e.g., an application) stored in a storagedevice (e.g., the storage device 130, the storage 320, and/or thestorage 490). In some embodiments, the processing device 120 (e.g., theprocessor 310 of the computing device 300, the CPU 440 of the mobiledevice 400, and/or one or more modules illustrated in FIG. 5) mayexecute the set of instructions and may accordingly be directed toperform the process 600.

In some embodiments, the process 600 may be used to perform aquantitative measurement on a subject (e.g., a patient, a specific organof the patient, a man-made object) using an MR scanner. The MR scannermay include one or more similar components to the MR scanner 110 asdescribed in connection with FIGS. 1 and 2. For example, the MR scannermay include a main magnet, three sets of gradient coils, an RF coil, orthe like, or any combination thereof. The three sets of gradient coilsmay be configured to generate magnetic gradient fields Gx, Gy, and Gz inthe X direction, the Y direction, and the Z direction defined by thecoordinate system 160, respectively.

In 601, the processing device 120 (e.g., the obtaining module 501) mayobtain a plurality of echo signals relating to the subject.

The plurality of echo signals may be excited by an MRI pulse sequenceapplied to the subject. For example, the processing device 120 may causethe MR scanner to apply the MRI pulse sequence on the subject so as toacquire the echo signals, and the processing device 120 may obtain theecho signals from the MR scanner. As another example, the echo signalsmay be previously acquired and stored in a storage device (e.g., thestorage device 130, the storage 320, an external storage device), andthe processing device 120 may retrieve the echo signals from the storagedevice.

In some embodiments, the MRI pulse sequence may include a CEST module, afat suppression module, an RF excitation pulse, one or more refocusingpulses, or the like, or any combination thereof. For illustrationpurposes, FIG. 7 illustrates an exemplary MRI pulse sequence 700according to some embodiments of the present disclosure. As shown inFIG. 7, the MRI pulse sequence 700 may include a CEST module, a fatsuppression module, an RF excitation pulse 701, and three or morerefocusing pulses (e.g., refocusing pulses 702, 703, and 704).

The CEST module may be configured to selectively excite exchangeableprotons or exchangeable molecules in the subject. As described elsewherein this disclosure, CEST imaging is a magnetic resonance imagingcontrast approach in which exogenous or endogenous compounds containingexchangeable protons or exchangeable molecules are selectively saturatedand after the transfer of this saturation, detected indirectly through awater signal with enhanced sensitivity. In MRI, saturation refers to anonequilibrium state with no net magnetization. In some embodiments, anRF saturation pulse may be used to produce the saturation, for example,in a certain region or a set of regions of the subject. For example, theCEST module may include a plurality of off-resonance Gaussian pulses orsquare-wave pulses, wherein each Gaussian pulse or square-wave pulse maylast several hundred milliseconds. For illustration purposes, FIG. 8provides an exemplary CEST module 800 according to some embodiments ofthe present disclosure. As shown in FIG. 8, the CEST module 800 mayinclude a train of RF saturation pulses and a plurality of spoilergradients. Each of the RF saturation pulses may be followed by a spoilergradient. The spoiler gradients may be used to reduce or minimizetransverse magnetization.

The fat suppression module may be applied between the CEST module andthe RF excitation pulse and configured to suppress signals from adiposetissue of the subject. For example, the fat suppression module may beimplemented through a water excitation pulse (e.g., a spectral-spatialwater-o11 my excitation pulse). Since fat and water protons resonate atdifferent frequencies, water protons may be selectively excited by awater excitation pulse. As another example, the fat suppression modulemay be implemented through a fat excitation pulse and a spoilergradient, wherein the fat excitation pulse may be configured toselectively excite fat protons and the spoiler gradient may beconfigured to dephase fat signals.

The RF excitation pulse may be applied after the fat suppression moduleand configured to excite a plurality of gradient echoes. For example,the RF excitation pulse may include a 90° RF excitation pulse. The countof the gradient echoes may be equal to any positive integer greater than1, such as 2, 3, 4, 5, or the like. For example, as shown in FIG. 7, Mgradient echoes S₁ to S_(M) excited by the RF excitation pulse may besequentially acquired at TE₁ to TE_(M), respectively. In someembodiments, the signal intensity of the plurality of gradient echoesmay decay with a time constant T2*. The plurality of gradient echoes maybe considered as T2*-weighted gradient echoes (or R2*-weighted gradientechoes). T2* may reflect an effect of a true T2 relaxation due tomolecular mechanisms as well as accelerated signal loss due to amagnetic field inhomogeneity. As described in FIG. 2, the T2 relaxationmay be the process by which transverse components of magnetization decayor dephase. It should be noted that, unless the context clearlyindicates otherwise, T2 and R2 are used interchangeably, and T2* and R2*are used interchangeably in the present disclosure.

The three or more refocusing pulses may be applied after the RFexcitation pulse, wherein each refocusing pulse may be configured toexcite one or more spin echoes. The spin echo(es) excited by arefocusing pulse may include a symmetric spin echo and/or an asymmetricspin echo. Generally, the signal intensity of a plurality of spin echoesexcited by a refocusing pulse may first increase and then decrease withtime. Among the spin echoes excited by a refocusing pulse, the spin echohaving the largest signal intensity may be referred to as a symmetricspin echo, and the remaining echo(es) may be referred to as asymmetricspin echo(es). In some embodiments, the one or more spin echoes excitedby a refocusing pulse may include a symmetric spin echo and one or moreasymmetric spin echoes. In some embodiments, the spin echo(es) excitedby a refocusing pulse may merely include a symmetric spin echo.

In some embodiments, a same type or different types of spin echoes maybe acquired after different refocusing pulses. For example, anasymmetric spin echo and a symmetric spin echo may be acquired after thefirst refocusing pulse, and only a symmetric spin echo may be acquiredafter each of the second refocusing pulse and third refocusing pulse. Asanother example, as shown in FIG. 7, the refocusing pulse 702 may excitea plurality of asymmetric spin echoes S_(M+1) to S_(2M) at TE_(M+1) toTE_(2M), respectively, and a symmetric spin echo SE₁ at TE_(SE1). Therefocusing pulse 703 may excite a plurality of asymmetric spin echoesS_(2M+1) to S_(3M) at TE_(2M+1) to TE_(3M), respectively, and asymmetric spin echo SE₂ at TE_(SE2). The refocusing pulse 704 may excitea plurality of asymmetric spin echoes S_(NM+1) to S_(NM+N) at TE_(NM+1)to TE_(NM+N), respectively, and a symmetric spin echo SE_(N) atTE_(SEN).

In some embodiments, a plurality of symmetric spin echoes may be excitedby the three or more refocusing pulses. The signal intensity of thesymmetric spin echoes may undergo an exponential decay with a timeconstant T2. The plurality of symmetric spin echoes may be considered asT2-weighted spin echoes (or R2-weighted spin echoes). In someembodiments, a plurality of asymmetric spin echoes may be excited by arefocusing pulse and acquired before a symmetric spin echo excited bythe refocusing pulse. The signal intensity of the asymmetric spin echoesmay undergo a T2 and T2* decay, and the asymmetric spin echoes may beconsidered as T2- and T2*-weighted spin echoes (or R2- and R2*-weightedspin echoes).

In some embodiments, an interval between the application of the RFexcitation pulse and the application of the first refocusing pulse amongthe refocusing pulses may be equal to half of an interval between theapplication of two consecutive refocusing pluses among the plurality ofrefocusing pulses. For example, in the MRI pulse sequence 700 as shownin FIG. 7, an interval between the RF excitation pulse 701 and therefocusing pulse 702 may be equal to half of an interval between therefocusing pulse 702 and the refocusing pulse 703.

As described above, both the gradient echoes excited by the RFexcitation pulse and the asymmetric spin echo(es) excited by therefocusing pulses may be T2- and T2*-weighted (or R2- and R2*-weighted)echoes, and the symmetric spin echo(es) excited by the refocusing pulsesmay be T2-weighted (or R2-weighted) echo(es). The gradient echoes andthe asymmetric spin echo(es) may be used to determine informationrelating to the T2* relaxation of the subject (e.g., a T2* value, a T2*decay curve, or an R2* value). The symmetric spin echo(es) may be usedto determine information relating to the T2 relaxation of the subject(e.g., a T2 value, a T2 decay curve, an R2 value). In some embodiments,a T2 decay curve may need to be determined based on three or moresymmetric spin echoes. For example, each of the three or more refocusingpulses may be configured to excite a symmetric spin echo, and the threeor more symmetric spin echoes may be used to determine the T2 decaycurve. Optionally, the three or more symmetric spin echoes may befurther used to determine an MWF value of the subject.

In some embodiments, the RF excitation pulse and the refocusing pulsesof the MRI pulse sequence may form a gradient and spin echo (GRASE)pulse sequence, e.g., a 2D GRASE pulse sequence or a 3D GRASE pulsesequence. A GRASE may be a hybrid sequence that combines gradient andspin echo sequences and can be used to acquire both gradient echoes andspin echoes. For example, a GRASE pulse sequence may be implemented by a90° RF excitation pulse followed by one or more 180° refocusing pulses.The GRASE imaging technique may have the characteristics of both the EPItechnique and the fast spin-echo imaging technique, and be used to imagethe subject with fast speed and an improved imaging accuracy. Forexample, compared with the EPI imaging technique, the GRASE imagingtechnique may generate an image with fewer image artifacts and smallerdeformation. In some embodiments, a 3D GRASE pulse sequence may be usedto generate a 3D MWF image relating to the brain of the subject byscanning the brain within a short time (e.g., several minutes). The 3DGRASE pulse sequence may include one or more phase encoding lobes alongthe Z direction, a 90° slab-selective RF excitation pulse and one ormore slab-selective refocusing pulses.

In some embodiments, the MRI pulse sequence may further include acrusher gradient including two lobes with the same polarity, wherein oneof the two lobes may be applied before a refocusing pulse and the otherone of the two lobes may be applied after the refocusing pulse. Thecrusher gradient may preserve a desired signal while eliminating orreducing an unwanted signal by manipulating the phase coherence of thetransverse magnetization. The crusher gradient may be used to reduce oreliminate spurious echoes (e.g., stimulated echoes) and avoid artifactsin a resulting image.

In some embodiments, a partial Fourier k-space technique may be appliedin the obtaining of the echo signals, so as to reduce the count of TEs(e.g., TE₁ to TE_(SEN)). In some embodiments, one or more techniques,such as a compressed sensing technique, a parallel imaging technique, aCartesian k-space trajectory acquisition technique, a non-Cartesiank-space trajectory acquisition technique, or the like, or anycombination thereof, may be applied during the MRI scan of the subjectin order to increase the count of echo signals acquired in the scan andimprove the data acquisition speed.

It should be noted that the MRI pulse sequence 700 is illustrated inFIG. 7 is merely provided for illustration, and not intended to belimiting. In some embodiments, one or more components of the MRI pulsesequence 700 described above may be omitted, and/or the MRI pulsesequence 700 may include one or more additional components. Merely byway of example, the fat suppression module may be omitted. The count ofa component of the MRI pulse sequence 700 may be adjusted according toactual needs. For example, only one gradient echo may be excited by theRF excitation pulse 701. As another example, the count of the refocusingpulses may be equal to 1 or 2.

In 602, the processing device 120 (e.g., the quantitative measurementmodule 502) may perform a quantitative measurement on the subject basedon the plurality of echo signals.

The quantitative measurement performed on the subject may includedetermining a quantitative parameter relating to the subject, generatinga quantitative map of the subject (which includes a value of aquantitative parameter of each physical point of the subject),generating a specific image reflecting a physiological property of thesubject, and/or any other measurement that can evaluate a characteristicof the subject. Exemplary quantitative parameters relating to thesubject may include a T1, a T2, a T2*, an R2, an R2*, an R2′, a B0field, a pH, an MWF, an APT, or the like, or any combination thereof.Exemplary quantitative maps of the subject may include a T1 map, a T2map, a T2* map, an R2 map, an R2* map, a B0 field distribution map, a pHmap, an MWF map, an APT map, or the like, or any combination thereof.Exemplary images of the subject may include a T1-weighted image, aT2-weighted image, a T2*-weighted image, an R2-weighted image, anR2*-weighted image, or the like, or any combination thereof.

By applying the MRI pulse sequence as described in 601, more informationrelating to the subject may be acquired during the MRI scan of thesubject, thereby achieving the acquisition of multiple quantitativeparameters of the subject through a single acquisition. For example,information relating to R2, R2*, MWF, APT, and pH may be obtainedthrough the single acquisition. As described elsewhere in the presentdisclosure, conventional CEST imaging methods usually need to performmultiple acquisitions to obtain a quantitative measurement result ofmultiple quantitative parameters. Through the process 600, theefficiency of medical imaging and disease diagnosis may be improved.

For illustration purposes, exemplary methods for performing quantitativemeasurement relating to a relaxation parameter (e.g., T1, T2, T2*, orT2′), MWF, APT, pH, and B0 of the brain of the subject are describedhereinafter. It should be noted that this is not intended to belimiting. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, the echo signals acquired in operation602 may be used to determine one or more other quantitative parameters.

In some embodiments, the processing device 120 may determine arelaxation parameter (e.g., T1, T2, T2*, or T2′) relating to the brainof the subject to access a microenvironment in the brain. Generally, thevalue of a specific relaxation parameter of a physical point in thebrain is within a certain range. If the value of the specific relaxationparameter is out of the certain range, the brain may be likely to have apathological change. Measurement result relating to a relaxationparameter of the brain may be used to detect a pathological change andprovide diagnostic information.

In some embodiments, the processing device 120 may determine an R2 valueand an R2* value relating to the subject based on the plurality of echosignals obtained in 601. As described in 601, the gradient echoesexcited by the RF excitation pulse may be R2*-weighted gradient echoes,the asymmetric spin echo(es) excited by the refocusing pulse(s) may beR2-and R2*-weighted spin echo(es), and the symmetric spin-echo(es)excited by the refocusing pulse(s) may be R2-weighted spin echo(es).Taking the MRI pulse sequence 700 as shown in FIG. 7 as an example, thegradient echoes S₁ to S_(M) may be R2*-weighted gradient echoes, theasymmetric spin echoes S_(M+1) to S_(2M) excited by the first refocusingpulse may be R2- and R2*-weighted spin echoes. The gradient echoes S₁ toS_(M) and the asymmetric spin echoes S_(M+1) to S_(2M) may be used todetermine the R2* value and the R2 value. Merely by way of example, theR2* value and the R2 value may be determined according to Equation (1),as below:

$\begin{matrix}{{{SI}(t)} = \left\{ {\begin{matrix}{{S_{0}^{1} \cdot e^{{- t} \cdot R_{2}^{*}}},{0 < t < \frac{{TE}_{SE}}{2}}} \\{{S_{0}^{2} \cdot e^{{- {TE}_{SE}} \cdot {({R_{2}^{*} - R_{2}})}} \cdot e^{{- t} \cdot {({{2 \cdot R_{2}} - R_{2}^{*}})}}},{\frac{{TE}_{SE}}{2} < t < {TE}_{SE}}}\end{matrix},} \right.} & (1)\end{matrix}$

where t refers to an echo time, SI(t) refers to the signal intensity ofan echo signal acquired at the echo time t, S₀ ¹ refers to the signalintensity of an initial signal that is used to model an exponentialsignal decay before the first refocusing pulse, S₀ ² refers to thesignal intensity of an initial signal that is used to model anexponential signal decay after the first refocusing pulse. S₀ ¹ and S₀ ²may be equilibrium signals and different from each other.

In some embodiments, an echo signal acquired within the interval

$0 < t < \frac{{TE}_{SE}}{2}$

may be a gradient echo, and an echo signal acquired within the interval

$\frac{{TE}_{SE}}{2} < t < {TE}_{SE}$

may be an asymmetric spin echo. In other words, a gradient echo excitedby the RF excitation pulse may be expressed as S₀ ¹.e^(−t.R*) ² , and anasymmetric spin echo excited by the first refocusing pulse may beexpressed as S₀ ².e^(−TE) ^(SE) ^(.(R*) ² ^(-R) ² ⁾. e^(−t-(2-R) ²^(-R*) ² ⁾. In some embodiments, the values of S₀ ¹ and S₀ ², the R2value, and the R2* value may be determined according to Equation (1)using a least square fitting algorithm. More descriptions regarding thedetermination of the R2 value and the R2* value may be found elsewherein the present disclosure. See. e.g., FIGS. 11, 12 and relevantdescriptions thereof.

In some embodiments, the processing device 120 may determine an R2′value relating to the subject based on the R2 value and the R2* value.The R2′ value may be equal to a difference between the R2* value and theR2 value, that is, R′₂=R*₂−R₂.

In some embodiments, the processing device 120 may determine acorrection parameter δ relating to a slice profile-induced mismatchbetween the gradient echoes before the first refocusing pulse and thespin echoes after the first refocusing pulse. Normally, a combination ofthe RF excitation pulse and the refocusing pulse with nonideal sliceprofile characteristics may lead to the slice profile-induced mismatch.This slice profile-induced mismatch may cause a signal magnitudedifference between the echo signals before and after the firstrefocusing pulse. The signal magnitude difference may increase as theslice profile-induced mismatch becomes more pronounced. To reduce oreliminate the effect of the signal magnitude difference, the correctionparameter 8 may be taken into consideration in the determination of theR2 value and R2* value and/or used to correct the R2 value and the R2*value. In some embodiments, the correction parameter 8 may be determinedaccording to Equation (1) through a least square fitting algorithm. Insome embodiments, if a count of the obtained echo signals is greaterthan a threshold, the correction parameter δ may be determined as aratio of S₀ ¹ and S₀ ², i.e.,

$\delta = {\frac{S_{0}^{1}}{S_{0}^{2}}.}$

In some embodiments, the processing device 120 may determine an MWFrelating to the subject. Myelin is critical for healthy brain function,and an accurate measurement of the myelin may be important forunderstanding the brain plasticity and detecting a neurodegenerativedisease. The MWF is a powerful quantitative parameter for accessing themyelin and other microstructure in the brain. For example, the MWF maybe defined as a ratio of an area in the T2 distribution arising frommyelin water to a total area of the entire T2 distribution. MWF may bevisually presented as an MWF map. The MWF map may be generated throughthe MR imaging technique, which may be used to visualize myelination inthe brain and the spinal cord in vivo.

In some embodiments, as described in connection with operation 601, theMRI pulse sequence may include three or more refocusing pulses, each ofwhich may excite a symmetric spin echo. The symmetric spin echoes (e.g.,the symmetric spin echoes SE₁, SE₂, . . . , SE_(N) as shown in FIG. 7)excited by the refocusing pulses may undergo an exponential decay with atime constant T2. The processing device 120 may determine an MWFrelating to the subject based on the symmetric spin echoes excited bythe at least three refocusing pulses. For example, the processing device120 may determine a T2 decay curve based on the symmetric spin echoes(e.g., SE₁, SE₂, and SE₃) using, for example, a fitting algorithm (e.g.,a Carr-Purcell-Meiboom-Gill (CPMG) or spin echo algorithm). Theprocessing device 120 may further determine the MWF relating to thesubject based on the T2 decay curve using a fitting algorithm (e.g., anon-negative least square (NNLS) algorithm). For example, the processingdevice 120 may construct an NNLS formula associated with the T2 decayand the MWF, and determine the MWF by solving the NNLS formula. The NNLSalgorithm may eliminate or reduce the effect of a stimulated echocontamination, thereby improving the accuracy of the MWF. In someembodiments, the count of the symmetric spin echoes used in thedetermination of the T2 decay curve may be associated with the accuracyof the determined T2 decay curve. The count of the spin echoes used todetermine the T2 decay curve may be determined according to a desireddetermination efficiency and a desired accuracy of the T2 decay curve.In some embodiments, the processing device 120 may determine the MWF ofeach physical point of the subject, and generate an MWF map thatincludes the MWF of each physical point of the subject.

In some embodiments, the processing device 120 may perform aquantitative measurement of an APT effect based on the echo signalsacquired in operation 601. The quantification of the APT effect may beused in detecting mobile proteins and peptides in biological tissues,and detecting certain diseases. For example, in a tumor region, theconcentration of proteins is elevated compared to surrounding tissues,and subsequently, the increased intracellular exchanges may lead to anincreased APT level.

In some embodiments, a magnetization transfer asymmetry (MTR_(asym))analysis may be used to quantify the APT effect. For example, the MRIpulse sequence may be applied to the subject multiple times withdifferent saturation frequencies of the CEST module in a plurality ofacquisitions. The plurality of echo signals acquired in operation 601may include a plurality of sets of echo signals, wherein each set ofecho signals may be acquired in one of the plurality of acquisitionswith a CEST module having a specific saturation frequency. Thesaturation frequency of a CEST module refers to the saturation frequencyof an RF saturation pulse of the CEST module. For example, the MRI pulsesequence may be applied to the subject 31 times with differentsaturation frequencies of the CEST module. The saturation frequenciesmay be within a range from +4.5 part per million (ppm) to −4.5ppm.Merely by way of example, the saturation frequencies may include +4.5ppm, +4.2 ppm, +3.9 ppm, 0 ppm, . . . , −3.9 ppm, −4.2 ppm, −4.5 ppm. Insome embodiments, the CEST module having a saturation frequency of 0 ppmmay be used to acquire a set of echo signals without RF saturation.

In some embodiments, an RF saturation effect on water in tissue may beplotted as a function of a saturation frequency offset relative to water(i.e., a z-spectrum). Ideally, the entire z-spectrum may besubstantially symmetric with respect to 0 ppm. Merely by way of example,it is assumed that the subject includes a cancer tissue and a healthytissue. As shown in FIGS. 13A to 13C, the horizontal axis represents thesaturation frequency of the RF saturation pulse, and the vertical axisrepresents a ratio of S_(sat) to SI₀. Ssat refers to a signal intensitymeasured with RF saturation, for example, excited by an MRI pulsesequence including both a CEST module and a fat suppression module. SI₀refers to a signal intensity measured without RF saturation, forexample, applied by an MRI pulse sequence including the fat suppressionmodule without the CEST module (i.e., a CEST with 0 ppm). In FIG. 13A, acurve 1310 marked with “+” may correspond to the cancer tissue, and acurve 1320 marked with “*” may correspond to the healthy tissue. FIG.13B was generated by folding the curves 1310 and 1320 along the dottedline corresponding 0 ppm. In FIG. 13B, a curve C1 corresponds to part ofthe curve 1310 in the range from 0 ppm to +4.5, a curve C2 correspondsto part of the curve 1310 in the range from −4.5 ppm to 0 ppm, a curveC3 corresponds to part of the curve 1320 in the range from 0 ppm to+4.5, and a curve C4 corresponds to part of the curve 1320 in the rangefrom −4.5 ppm to 0 ppm. The processing device 120 may further generate acurve C5 by determining a difference between the curves C1 and C2, andgenerate a curve C6 by determining a difference between the curves C3and C4. As shown in FIG. 13C, at a same saturation frequency level, thedifference value corresponding to the cancer tissue in the curve C5 ishigher than the difference value corresponding to the healthy tissue inthe curve c6. By analyzing the z-spectrum of a tissue, the processingdevice 120 may determine whether the tissue is a cancer tissue or ahealthy tissue.

In some embodiments, the processing device 120 may perform the MTRasymanalysis according to Equation (2) as below:

$\begin{matrix}{{{{MTR}_{asym}(\omega)} = \frac{{S\left( {- \omega} \right)} - {S(\omega)}}{{SI}_{0}}},} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

where ω and −ω refer to saturation frequencies, S(ω) refers to thesignal intensity of an echo signal of bulk water when a CEST module withthe saturation frequency ω is applied, S(−ω) refers to the signalintensity of an echo signal of bulk water when a CEST module with thesaturation frequency −ω is applied, and SI₀ refers to the signalintensity of an echo signal of bulk water when no RF saturation isapplied. The introduction of SI₀ may be used to reduce or eliminate theeffect of a magnetization transfer and a direct water saturation.

In some embodiments, the processing device 120 may determine a pH valuerelating to the subject. pH information of the subject may serve as abasis for detecting some diseases, such as a stroke, a kidney failure, atumor. For example, acidosis may be caused by various diseases. A quickand accurate detection of an acid-base change of a tissue may havescientific significance and clinical value for early detection andevaluation of a disease. Merely by way of example, an extracellular pHof a normal cell may be around 7.4, and an intracellular pH of thenormal cell may be around 7.3. An extracellular pH of a tumor cell maybe 6.0 to 7.0, and an intracellular pH of the tumor cell may be greaterthan 7.4. The pH value of a cell may be used to detect a tumor.

In some embodiments, the processing device 120 may determine a pH valuerelating to the subject using one or more techniques for PH measurement,such as a water proton nuclear magnetic resonance (NMR) via CEST withselected chemical exchange sites technique, an amine and amideconcentration-independent detection (AACID) technique, a water-exchangespectroscopy (WEX) technique, or the like. In some embodiments, theprocessing device 120 may determine a pH value according to Equation(3), as below:

k _(b) =k ₀ +k _(base)* 10^(−(14-pH)),   Equation (3)

wherein k₀ refers to a default exchange rate, k_(b) refers to anexchange rate between amine protons and bulk water protons, and k_(base)refers to a base-catalyzed exchange rate constant.

In some embodiments, k_(b) may be associated with a result of theMTRasym analysis as aforementioned. For example, k_(b) may be determinedaccording to Equation (4) as below:

$\begin{matrix}{{k_{b} = \frac{R_{1W}}{f_{S} \cdot \left\lbrack {\frac{\alpha \cdot \left( {1 - \sigma} \right)}{{{MTR}_{asym}(\omega)} - {\Delta \; {MTR}\; {\prime_{asym}(\omega)}}} - 1} \right\rbrack}},} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

wherein R_(1W) refers to a longitudinal relaxation rate of water, αrefers to a saturation efficiency, σ refers to an overflow element,f_(s) refers to a molality ratio of exchangeable protons, andΔMTR′_(asym)(ω) refers to a change in a signal caused by a magnetizationtransfer (MT) effect. Based on Equations (3) and (4), the pH value maybe determined.

In some embodiments, the processing device 120 may generate a B0 mapbased on the plurality of echo signals or a portion of the echo signals(e.g., any two echo signals with different TEs). For example, theprocessing device 120 may generate the B0 map using a water saturationshift referencing (WASSR) method. Optionally, the processing device 120may use the B0 map to correct one or more other quantitative parametersrelating to the subject, such as APT.

Generally, an accurate and efficient disease diagnosis may need variousinformation for comprehensive judgment. Different quantitativeparameters, quantitative maps, and images may provide different medicalinformation, for example, anatomical information, acid-base balanceinformation, metabolite information, blood flow and blood vesselinformation, brain function information, cardiac function information,or the like, or any combination thereof. The MRI pulse sequence used inthe present disclosure may be used to acquire information relating tovarious quantitative parameters (e.g., R2, R2*, MWF, APT, and pH)simultaneously and efferently. This may improve the efficiency ofdiseases diagnose. In addition, in some embodiments, a B0 correction maybe performed to improve the accuracy of the measurement result.

It should be noted that the above description regarding the process 600is merely provided for the purposes of illustration, and not intended tolimit the scope of the present disclosure. For persons having ordinaryskills in the art, multiple variations and modifications may be madeunder the teachings of the present disclosure. However, those variationsand modifications do not depart from the scope of the presentdisclosure. In some embodiments, the process 600 may be accomplishedwith one or more additional operations not described and/or without oneor more of the operations discussed above. For example, the process 600may further include an operation to send a quantitative measurementresult to a terminal 140 for display.

FIG. 9 is a schematic diagram illustrating an exemplary GRASE pulsesequence 900 according to some embodiments of the present disclosure.The GRASE pulse sequence 900 may be an exemplary embodiment of the MRIpulse sequence 700 as illustrated in FIG. 7. As shown in FIG. 9, theGRASE pulse sequence 900 may include a CEST module, a fat suppressionmodule, a 90° RF excitation pulse, and three or more 180° refocusingpulses. The 90° RF excitation pulse of the GRASE pulse sequence 900 mayexcite a plurality of gradient echoes S₁ to S_(M) at TE₁ to TE_(M),respectively. The first 180° refocusing pulse of the GRASE pulsesequence 900 may excite two or more asymmetric spin echoes includingS_(M+1) at TE_(M+1) and S_(M+2) at TE_(M+2), and a symmetric spin echoSE₁ at TE_(SE1). The second 180° refocusing pulse of the GRASE pulsesequence 900 may excite two or more asymmetric spin echoes includingS_(2M+1) at TE_(2M+1) and S_(2M+2) at TE_(2M+2), and a symmetric spinecho SE₂ at TE_(SE2). The third 180° refocusing pulse of the GRASE pulsesequence 900 may excite two or more asymmetric spin echoes includingS_(3M+1) at TE_(3M+1) and S_(3M+2) at TE_(3M+2), and a symmetric spinecho SE₃ at TE_(SE3).

FIG. 10 is a schematic diagram illustrating an exemplary GRASE pulsesequence 1000 according to some embodiments of the present disclosure.The GRASE pulse sequence 1000 may be another exemplary embodiment of theMRI pulse sequence 700 as illustrated in FIG. 7. As shown in FIG. 10,the GRASE pulse sequence 1000 may include a CEST module, a 90° RFexcitation pulse, and by two 180° refocusing pulses. The 90° RFexcitation pulse of the GRASE pulse sequence 1000 may excite threegradient echoes S₁ to S₃ at TE₁ to TE₃, respectively. The first 180°refocusing pulse of the GRASE pulse sequence 1000 may excite threeasymmetric spin echoes S₄ and S₅ at TE₄ to TE₆, and a symmetric spinecho SE₁ at TE_(SE1).

FIG. 11 is a schematic diagram illustrating an exemplary GRASE pulsesequence 1100 according to some embodiments of the present disclosure.The GRASE pulse sequence 1100 may be another exemplary embodiment of theMRI pulse sequence 700 as illustrated in FIG. 7. As shown in FIG. 11,the GRASE pulse sequence 1100 may include a CEST module, a fatsuppression module, a 90° RF excitation pulse, and a 180° refocusingpulse. The 90° RF excitation pulse of the GRASE pulse sequence 1100 mayexcite a gradient echo S₁ at TE₁ and a gradient echo S₂ at TE₂. The 180°refocusing pulse of the GRASE pulse sequence 1100 may excite anasymmetric spin echoes S₃ at TE₃ and a symmetric spin echo SE₁ (alsodenoted as S₄) at TE_(SE1) (also denoted as TE₄).

The processing device 120 may determine an R2 value, an R2* value, andan R2′ value relating to the subject based on the echo signals S₁-S₄.For example, the signal intensity (or amplitude) of the echo signalsS₁-S₄ may be represented as Equations (5) to (8), respectively, asbelow:

SI ₁ =S(TE ₁)=S ₀ ¹ .e ^(−TE) ¹ ^(.R*) ² ,   Equation (5)

SI ₂ =S(TE ₂)=S ₀ ¹ .e ^(−TE) ² ^(.R*) ² ,   Equation (6)

SI ₃ =S(TE ₃)=S ₀ ² .e ^((−TE) ⁴ ^(+TE) ³ ^().R*) ² .e ^((TE) ⁴ ^(-2.TE)³ ^().R) ²⁾ ,   Equation (7)

SI ₄ =S(TE ₄)=S ₀ ² .e ^(−TE) ⁴ ^(.R) ² ,   Equation (8)

where SI_(n)(n=1, 2, 3, 4) represent the signal intensity of the echosignal Sn.

The R2 value and the R2* value relating to the subject may be determinedaccording to Equation (9) as below:

$\begin{matrix}{{A = {B^{- 1}S}},{{{where}\mspace{14mu} A} = \begin{pmatrix}{\ln \mspace{14mu} \left( {SI}_{0} \right)} \\{\ln \mspace{14mu} (\delta)} \\{\ln \mspace{14mu} \left( R_{2}^{*} \right)} \\{\ln \mspace{14mu} \left( R_{2} \right)}\end{pmatrix}},{B = \begin{pmatrix}1 & 0 & {- {TE}_{1}} & 0 \\1 & 0 & {- {TE}_{2}} & 0 \\1 & {- 1} & {{- {TE}_{4}} + {TE}_{3}} & {{TE}_{4} - {2 \cdot {TE}_{3}}} \\1 & {- 1} & 0 & {- {TE}_{4}}\end{pmatrix}},{S = \begin{pmatrix}{\ln \mspace{14mu} \left( {SI}_{1} \right)} \\{\ln \mspace{14mu} \left( {SI}_{2} \right)} \\{\ln \mspace{14mu} \left( {SI}_{3} \right)} \\{\ln \mspace{14mu} \left( {SI}_{4} \right)}\end{pmatrix}},} & (9)\end{matrix}$

SI₀ refers to a signal intensity measured without RF saturation asdescribed in 602, and δ represents the correction parameter relating toa slice profile-induced mismatch as described in 602.

The R2 value and the R2* value may be determined according to Equations(5)-(9). The R′ value relating to the subject may be determinedaccording to Equation (10) as below:

R′ ₂ =R* ₂ −R ₂.   (10)

In some embodiments, the processing device 120 may determine the R2value of each physical point of the subject, and generate an R2 map thatincludes the R2 value of each physical point of the subject.Additionally or alternatively, the processing device 120 may determinethe R2* value of each physical point of the subject, and generate an R2*map that includes the R2* value of each physical point of the subject.Additionally or alternatively, the processing device 120 may determinethe R2′ value of each physical point of the subject, and generate an R2′map that includes the R2′ value of each physical point of the subject.

FIG. 12 is a schematic diagram illustrating an exemplary GRASE pulsesequence 1200 according to some embodiments of the present disclosure.The GRASE pulse sequence 1200 may be another exemplary embodiment of theMRI pulse sequence 700 as illustrated in FIG. 7. As shown in FIG. 12,the GRASE pulse sequence 1200 may include a CEST module, a 90° RFexcitation pulse, and two 180° refocusing pulses. The 90° RF excitationpulse of the GRASE pulse sequence 1200 may excite three gradient echoesS₁, S₂, and S₃ at TE₁, TE₂, and TE₃. The first 180° refocusing pulse ofthe GRASE pulse sequence 1200 may excite two asymmetric spin echoes S₄and S₅ at TE₄ and TE₅, and a symmetric spin echo SE₁ (also denoted asS₆) at TE_(SE1) (also denoted as TE₆).

The processing device 120 may determine an R2 value, an R2* value, andan R2′ value relating to the subject based on the echo signals S₁-S₆.For example, the signal intensity (or amplitude) of the echo signalsS₁-S₆ may be represented as Equations (11) to (16), respectively, asbelow:

SI ₁ =S(TE ₁)=S ₀ ¹ .e ^(−TE) ¹ ^(.R*) ² ,   (11)

SI ₂ =S(TE ₂)=S ₀ ¹ .e ^(−TE) ² ^(.R*) ² ,   (12)

SI ₃ =S(TE ₃)=S ₀ ¹ .e ^(−TE) ³ ^(.R*) ² ,   (13)

SI ₄ =S(TE ₄)=S ₀ ² .e ^((TE) ⁴ ^(-TE) ⁶ ^().R*) ² .e ^((TE) ⁶ ^(-2.TE)⁴ ^().R) ₂ ⁾,   (14)

SI ₅ =S(TE ₅)=S ₀ ² .e ^((TE) ⁵ ^(-TE) ⁶ ^().R*) ² .e ^((TE) ⁶ ^(-2.TE)⁵ ^().R) ² ⁾,   (15)

SI ₆ =S(TE ₆)=S ₀ ² .e ^(−TE) ⁶ ^(.R) ² ,   (16)

where SI_(n)(n=1, 2, . . . , 6) represent the signal intensity of theecho signal Sn.

The R2 value and the R2* value relating to the subject may be determinedaccording to Equation (17) as below:

A=B⁻¹S,   (17)

where

${A = \begin{pmatrix}{\ln \mspace{14mu} \left( {SI}_{0} \right)} \\0 \\0 \\{\ln \mspace{14mu} (\delta)} \\{\ln \mspace{14mu} \left( R_{2}^{*} \right)} \\{\ln \mspace{14mu} \left( R_{2} \right)}\end{pmatrix}},{B = \begin{pmatrix}1 & 0 & 0 & 0 & {- {TE}_{1}} & 0 \\1 & 0 & 0 & 0 & {- {TE}_{2}} & 0 \\1 & 0 & 0 & 0 & {- {TE}_{3}} & 0 \\1 & 0 & 0 & {- 1} & {{TE}_{4} - {TE}_{6}} & {{TE}_{6} - {2 \cdot {TE}_{4}}} \\1 & 0 & 0 & {- 1} & {{TE}_{5} - {TE}_{6}} & {{TE}_{6} - {2 \cdot {TE}_{5}}} \\1 & 0 & 0 & {- 1} & 0 & {- {TE}_{6}}\end{pmatrix}},{{{and}\mspace{14mu} S} = {\begin{pmatrix}{\ln \mspace{14mu} \left( {SI}_{1} \right)} \\{\ln \mspace{14mu} \left( {SI}_{2} \right)} \\{\ln \mspace{14mu} \left( {SI}_{3} \right)} \\{\ln \mspace{14mu} \left( {SI}_{4} \right)} \\{\ln \mspace{14mu} \left( {SI}_{5} \right)} \\{\ln \mspace{14mu} \left( {SI}_{6} \right)}\end{pmatrix}.}}$

In some embodiments, the R2 value and the R2* value relating to thesubject may be determined without the echo signals S₃ and S₄, wherein

${A = \begin{pmatrix}{\ln \mspace{14mu} \left( {SI}_{0} \right)} \\{\ln \mspace{14mu} (\delta)} \\{\ln \mspace{14mu} \left( R_{2}^{*} \right)} \\{\ln \mspace{14mu} \left( R_{2} \right)}\end{pmatrix}},{B = \begin{pmatrix}1 & 0 & {- {TE}_{1}} & 0 \\1 & 0 & {- {TE}_{2}} & 0 \\1 & {- 1} & {{- {TE}_{5}} - {TE}_{6}} & {{TE}_{6} - {2 \cdot {TE}_{5}}} \\1 & {- 1} & 0 & {- {TE}_{6}}\end{pmatrix}},{S = {\begin{pmatrix}{\ln \mspace{14mu} \left( {SI}_{1} \right)} \\{\ln \mspace{14mu} \left( {SI}_{2} \right)} \\{\ln \mspace{14mu} \left( {SI}_{5} \right)} \\{\ln \mspace{14mu} \left( {SI}_{6} \right)}\end{pmatrix}.}}$

The R2 value and the R2* value may be determined according to Equations(11)-(17). The corresponding R′ value relating to the subject may bedetermined according to Equation (10). Similar to the embodiment asdescribed in connection with FIG. 11, the processing device 120 maygenerate an R2 map, an R2* map, and/or an R2′ map relating to thesubject.

It should be noted that the above exemplary pulse sequences illustratedin FIGS. 9-12 and the descriptions thereof are merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations and modifications may be made under the teachings ofthe present disclosure. However, those variations and modifications donot depart from the scope of the present disclosure. In someembodiments, a GRASE pulse sequence may include a rewinding gradientapplied along a slice-encoding direction after a readout of an echosignal, so as to eliminate or reduce an effect of a phase modulationgradient applied in an acquisition step for spatial encoding along aphase-encoding direction. Moreover, the Equations provided above areillustrative examples and can be modified in various ways.

Having thus described the basic concepts, it may be rather apparent tothose skilled in the art after reading this detailed disclosure that theforegoing detailed disclosure is intended to be presented by way ofexample only and is not limiting. Various alterations, improvements, andmodifications may occur and are intended to those skilled in the art,though not expressly stated herein. These alterations, improvements, andmodifications are intended to be suggested by this disclosure, and arewithin the spirit and scope of the exemplary embodiments of thisdisclosure.

Moreover, certain terminology has been used to describe embodiments ofthe present disclosure. For example, the terms “one embodiment,” “anembodiment,” and/or “some embodiments” mean that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the present disclosure.

Further, it will be appreciated by one skilled in the art, aspects ofthe present disclosure may be illustrated and described herein in any ofa number of patentable classes or context including any new and usefulprocess, machine, manufacture, or composition of matter, or any new anduseful improvement thereof. Accordingly, aspects of the presentdisclosure may be implemented entirely hardware, entirely software(including firmware, resident software, micro-code, etc.) or combiningsoftware and hardware implementation that may all generally be referredto herein as a “unit,” “module,” or “system.” Furthermore, aspects ofthe present disclosure may take the form of a computer program productembodied in one or more computer readable media having computer readableprogram code embodied thereon.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including electro-magnetic, optical, or thelike, or any suitable combination thereof. A computer readable signalmedium may be any computer readable medium that is not a computerreadable storage medium and that may communicate, propagate, ortransport a program for use by or in connection with an instructionexecution system, apparatus, or device. Program code embodied on acomputer readable signal medium may be transmitted using any appropriatemedium, including wireless, wireline, optical fiber cable, RF, or thelike, or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of thepresent disclosure may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Scala, Smalltalk, Eiffel, JADE, Emerald, C++, C#, VB. NET,Python or the like, conventional procedural programming languages, suchas the “C” programming language, Visual Basic, Fortran 2103, Perl, COBOL2102, PHP, ABAP, dynamic programming languages such as Python, Ruby andGroovy, or other programming languages. The program code may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider) or in a cloud computing environment or offered as aservice such as a Software as a Service (SaaS).

Furthermore, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations therefore, is notintended to limit the claimed processes and methods to any order exceptas may be specified in the claims. Although the above disclosurediscusses through various examples what is currently considered to be avariety of useful embodiments of the disclosure, it is to be understoodthat such detail is solely for that purpose, and that the appendedclaims are not limited to the disclosed embodiments, but, on thecontrary, are intended to cover modifications and equivalentarrangements that are within the spirit and scope of the disclosedembodiments. For example, although the implementation of variouscomponents described above may be embodied in a hardware device, it mayalso be implemented as a software only solution, for example, aninstallation on an existing server or mobile device.

Similarly, it should be appreciated that in the foregoing description ofembodiments of the present disclosure, various features are sometimesgrouped together in a single embodiment, figure, or description thereoffor the purpose of streamlining the disclosure aiding in theunderstanding of one or more of the various inventive embodiments. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed object matter requires more features than areexpressly recited in each claim. Rather, inventive embodiments lie inless than all features of a single foregoing disclosed embodiment.

In some embodiments, the numbers expressing quantities or propertiesused to describe and claim certain embodiments of the application are tobe understood as being modified in some instances by the term “about,”“approximate,” or “substantially.” For example, “about,” “approximate,”or “substantially” may indicate ±1%, ±5%, ±10%, or ±20% variation of thevalue it describes, unless otherwise stated. Accordingly, in someembodiments, the numerical parameters set forth in the writtendescription and attached claims are approximations that may varydepending upon the desired properties sought to be obtained by aparticular embodiment. In some embodiments, the numerical parametersshould be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof some embodiments of the application are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspracticable.

Each of the patents, patent applications, publications of patentapplications, and other material, such as articles, books,specifications, publications, documents, things, and/or the like,referenced herein is hereby incorporated herein by this reference in itsentirety for all purposes, excepting any prosecution file historyassociated with same, any of same that is inconsistent with or inconflict with the present document, or any of same that may have alimiting effect as to the broadest scope of the claims now or laterassociated with the present document. By way of example, should there beany inconsistency or conflict between the description, definition,and/or the use of a term associated with any of the incorporatedmaterial and that associated with the present document, the description,definition, and/or the use of the term in the present document shallprevail.

In closing, it is to be understood that the embodiments of theapplication disclosed herein are illustrative of the principles of theembodiments of the application. Other modifications that may be employedmay be within the scope of the application. Thus, by way of example, butnot of limitation, alternative configurations of the embodiments of theapplication may be utilized in accordance with the teachings herein.Accordingly, embodiments of the present application are not limited tothat precisely as shown and described.

What is claimed is:
 1. A system for magnetic resonance imaging (MRI),comprising: at least one storage device including a set of instructions;and at least one processor configured to communicate with the at leastone storage device, wherein when executing the set of instructions, theat least one processor is configured to direct the system to performoperations including: obtaining a plurality of echo signals relating toa subject, the plurality of echo signals being excited by an MRI pulsesequence applied to the subject; and performing a quantitativemeasurement on the subject based on the plurality of echo signals,wherein the MRI pulse sequence comprises: a chemical exchange andsaturation transfer (CEST) module configured to selectively exciteexchangeable protons or exchangeable molecules in the subject; a radiofrequency (RF) excitation pulse applied after the CEST module configuredto excite a plurality of gradient echoes; and one or more refocusingpulses applied after the RF excitation pulse, wherein each of the one ormore refocusing pulses is configured to excite one or more spin echoes,and the one or more spin echoes excited by at least one of the one ormore refocusing pulses comprise a symmetric spin echo and one or moreasymmetric spin echoes.
 2. The system of claim 1, wherein the one ormore refocusing pulses comprise at least three refocusing pulses.
 3. Thesystem of claim 2, wherein the one or more spin echoes excited by eachof the at least three refocusing pulses comprise a symmetric spin echo,and the performing a quantitative measurement on the subject based onthe plurality of echo signals comprises: determining a Myelin waterfraction (MWF) relating to the subject based on the symmetric spinechoes excited by the at least three refocusing pulses.
 4. The system ofclaim 1, wherein the performing a quantitative measurement on thesubject based on the plurality of echo signals comprises: determining anR2 value and an R2* value relating to the subject based on the pluralityof gradient echoes, the one or more asymmetric spin echoes excited bythe at least one refocusing pulse, and the symmetric spin echo excitedby the at least one refocusing pulse.
 5. The system of claim 4, whereinthe performing a quantitative measurement on the subject based on theplurality of echo signals further comprises: determining an R2′ valuerelating to the subject based on the R2 value and the R2* value.
 6. Thesystem of claim 1, wherein the at least one processor is furtherconfigured to direct the system to perform the operations including:determining a coefficient that measures a difference between a sliceprofile corresponding to the RF excitation pulse and a slice profilecorresponding to the at least one refocusing pulse based on theplurality of gradient echoes, the one or more asymmetric spin echoesexcited by the at least one refocusing pulse, and the symmetric spinecho excited by the at least one refocusing pulse.
 7. The system ofclaim 1, wherein plurality of gradient echoes are R2*-weighted gradientechoes, or more asymmetric spin echoes excited by the at least onerefocusing pulse are R2- and R2*-weighted spin echoes, and symmetricspin-echo excited by the at least one refocusing pulse is an R2-weightedspin echo.
 8. The system of claim 1, wherein the MRI pulse sequencefurther comprises a fat suppression module applied between the CESTmodule and the RF excitation pulse configured to suppress signals froman adipose tissue of the subject.
 9. The system of claim 1, wherein MRIpulse sequence is applied to the subject for multiple times withdifferent saturation frequencies of the CEST module in a plurality ofacquisitions, plurality of echo signals comprise a plurality of sets ofecho signals each of which are acquired in one of the plurality ofacquisitions, and performing a quantitative measurement on the subjectbased on the plurality of echo signals comprises: p1 performing amagnetization transfer asymmetry (MTRasym) analysis on the plurality ofsets of echo signals.
 10. The system of claim 9, wherein the performinga quantitative measurement on the subject based on the plurality of echosignals further comprises: determining a pH value relating to thesubject based on a result of the MTRasym analysis.
 11. The system ofclaim 1, the at least one processor is further configured to direct thesystem to perform the operations including: generating a B0 map based onthe plurality of echo signals.
 12. A method for magnetic resonanceimaging (MRI), comprising: obtaining a plurality of echo signalsrelating to a subject, the plurality of echo signals being excited by anMRI pulse sequence applied to the subject; and performing a quantitativemeasurement on the subject based on the plurality of echo signals,wherein the MRI pulse sequence comprises: a chemical exchange andsaturation transfer (CEST) module configured to selectively exciteexchangeable protons or exchangeable molecules in the subject; a radiofrequency (RF) excitation pulse applied after the CEST module configuredto excite a plurality of gradient echoes; and one or more refocusingpulses applied after the RF excitation pulse, wherein each of the one ormore refocusing pulses is configured to excite one or more spin echoes,and the one or more spin echoes excited by at least one of the one ormore refocusing pulses comprise a symmetric spin echo and one or moreasymmetric spin echoes.
 13. The method of claim 12, wherein the one ormore refocusing pulses comprise at least three refocusing pulses. 14.The method of claim 13, wherein the one or more spin echoes excited byeach of the at least three refocusing pulses comprise a symmetric spinecho, and the performing a quantitative measurement on the subject basedon the plurality of echo signals comprises: determining a Myelin waterfraction (MWF) relating to the subject based on the symmetric spinechoes excited by the at least three refocusing pulses.
 15. The methodof claim 12, wherein the performing a quantitative measurement on thesubject based on the plurality of echo signals comprises: determining anR2 value and an R2* value relating to the subject based on the pluralityof gradient echoes, the one or more asymmetric spin echoes excited bythe at least one refocusing pulse, and the symmetric spin echo excitedby the at least one refocusing pulse.
 16. The method of claim 12,wherein the plurality of gradient echoes are R2*-weighted gradientechoes, one or more asymmetric spin echoes excited by the at least onerefocusing pulse are R2- and R2*-weighted spin echoes, and the symmetricspin-echo excited by the at least one refocusing pulse is an R2-weightedspin echo.
 17. The method of claim 12, wherein the MRI pulse sequence isapplied to the subject for multiple times with different saturationfrequencies of the CEST module in a plurality of acquisitions, theplurality of echo signals comprise a plurality of sets of echo signalseach of which are acquired in one of the plurality of acquisitions, andthe performing a quantitative measurement on the subject based on theplurality of echo signals comprises: performing a magnetization transferasymmetry (MTRasym) analysis on the plurality of sets of echo signals.18. The method of claim 17, wherein the performing a quantitativemeasurement on the subject based on the plurality of echo signalsfurther comprises: determining a pH value relating to the subject basedon a result of the MTRasym analysis.
 19. The method of claim 12, furthercomprising: generating a B0 map based on the plurality of echo signals.20. A non-transitory readable medium, comprising at least one set ofinstructions for magnetic resonance imaging (MRI), wherein when executedby at least one processor, the at least one set of instructions directsthe at least one processor to perform a method, the method comprising:obtaining a plurality of echo signals relating to a subject, theplurality of echo signals being excited by an MRI pulse sequence appliedto the subject; and performing a quantitative measurement on the subjectbased on the plurality of echo signals, wherein the MRI pulse sequencecomprises: a chemical exchange and saturation transfer (CEST) moduleconfigured to selectively excite exchangeable protons or exchangeablemolecules in the subject; a radio frequency (RF) excitation pulseapplied after the CEST module configured to excite a plurality ofgradient echoes; and one or more refocusing pulses applied after the RFexcitation pulse, wherein each of the one or more refocusing pulses isconfigured to excite one or more spin echoes, and the one or more spinechoes excited by at least one of the one or more refocusing pulsescomprise a symmetric spin echo and one or more asymmetric spin echoes.